In 3GPP Long Term Evolution (LTE), a user equipment (UE) can be configured with dual connectivity (DC), where the UE is connected to two separate eNBs, associated via separate Medium Access Control (MAC) entities/cell groups. In the uplink (UL) split bearer configuration of DC, the UE maintains one Packet Data Convergence Protocol (PDCP) entity that routes the data via two separate Radio Link Control (RLC) entities, via the two cell groups to the two eNBs.
When a UE is configured with DC and UL split bearer, the UE is configured with two UL transmission paths associated with two separate RLC entities. Transmission on these transmission paths is triggered by reception of an UL transmission grant from the eNB for the respective path. In LTE, the PDCP entity delivers PDCP Protocol Data Units (PDUs) to the RLC entity for transmission when a transmission opportunity is indicated by lower layers, or when requested by lower layers for this path (i.e., upon grant reception). A PDCP PDU is then delivered to the RLC entity and the RLC entity builds an RLC PDU. This implies that PDCP stores the PDCP PDUs and does not deliver them to lower layers until requested by them. The RLC state variables are only updated when PDCP delivers a PDCP PDU to the RLC entity.
When the PDCP data volume is above a configured split threshold, the UE reports that data as available for transmission to both eNBs, otherwise the report is only towards a configured prioritized eNB (i.e., for a single prioritized path). In both cases, the network may then issue grants independently to each of the paths. This behavior allows the network to control the load that each of the paths carries. This is currently specified as a procedure to report uplink data available at PDCP to a Master Cell Group (MCG) and Secondary Cell Group (SCG) when a pre-configured threshold amount of data is exceeded.
In 3GPP New Radio (NR), the PDCP entity can deliver a PDCP PDU to the RLC entity at any point in time and the RLC entity can also build an RLC PDU at any point in time, even before a transmission opportunity is indicated by lower layers. This means that the UE pre-selects the path in which the PDCP PDUs are placed regardless of whether the UE has a grant or not in that path. In contrast to LTE, the UE in NR reports part of the available data to the first path, part of the other data to the second path, and may still report part of the data that was not yet delivered to one of the two RLC entities to both paths.